1. Field of the Invention
This invention relates in general to hydraulic actuator devices and more particularly to such devices which hydraulically drive linear or rotary actuators.
2. Description of the Prior Art
It has long been recognized that hydraulic, as opposed to purely mechanical or electromechanical actuation is more desireable for certain applications. One reason for this is that hydraulic systems have been found more practical in applications requiring high reliability and large force/velocity capability combined with rapid response. For example, a majority of commercial and military aircraft today use hydraulic actuation for their primary flight control surfaces. However, hydraulic servoactuation has certain limitation, foremost of which is the need for a central hydraulic supply system. A hydraulic pump is required, together with a prime mover to drive the pump, a reservoir, an accumulator, piping to convey the hydraulic pressure to each remotely located servoactuator, etc. There is considerable cost and installation expense, potential maintenance problems due to leakage from the piping, substantial energy losses at the pump, undesirable noise, and for aircraft installations considerable weight and bulk of hardware.
There have been many attempts to replace hydraulic servoactuation systems with electromechanical servoactuation systems, thereby eliminating the central hydraulic supply system. These attempts have accelerated, due to recent development in servomotors using rare earth permanent magnets, and recent developments in the electronic control hardware that such motors require. However, the necessary gearing (and often clutches) between such improved electric motors and the load have emerged as the weak link, and have not improved to the degree necessary to replace hydraulic servoactuation in many applications.
The present invention uses electric motor actuation rather than a central hydraulic supply, but substitutes a self-contained hydraulic transmission for the mechanical transmission. This avoids many of the problems which have not been able to be solved in the mechanical clutches and gears. For example, the present invention can provide an effective gear ratio of 2,000 to 1 or higher between the motor and load, without using any gears. This eliminates the gear tooth fatigue problems encountered in electromechanical servoactuators. The need for clutches in redundant mechanical systems is eliminated; a failed servoactuator constructed in accordance with the present invention can be backdriven by other parallel servoactuators.
Another problem that has long plagued hydraulic systems is leakage. Leakage is essentially eliminated in the present invention by means of a design which provides only one possible leakpoint, rather than the many such potential leakpoints in the prior art. This results in greatly reduced maintenance expense.
Still another problem of prior hydraulic systems is filtration of the fluid. Conventional filtration of the hydraulic fluid is not possible where the flow of the hydraulic fluid is not unidirectional. Flow reversals sweep out contaminant particles created by pump wear. The present invention provides a filtration design which solves this problem, assuring long life for the servoactuator.
In the present invention, the electric motor drives the hydraulic pump on a demand basis, generating only the required pressure and flow. Compared to the prior art, this conserves energy, reduces electrical power costs, and also generates less noise (important in industrial applications). This creates a drive of high efficiency. Still further, the present invention provides self contained failure detection capabilities to reduce maintenance costs.